1. Field of the Invention
The present invention generally relates to communication systems, and more particularly, to a system and method for to canceling crosstalk in a communications system.
2. Discussion of the Related Art
In recent years telecommunication systems have expanded from traditional POTS communications to include high-speed data communications as well. As is known, POTS communications includes not only the transmission of voice information, but also PSTN (public switched telephone network) modem information, control signals, and other information that is transmitted in the POTS bandwidth, which extends from approximately 300 hertz to approximately 3.4 kilohertz.
New, high-speed data communications provided over digital subscriber lines (DSL), such as Asymmetric Digital Subscriber Line (ADSL), Rate Adaptive Digital Subscriber Line (RADSL), High-Speed Digital Subscriber Line (HDSL), etc. (more broadly denoted as xDSL) provide for high speed data transmissions, as is commonly used in communicating over the Internet. As is known, the bandwidth for xDSL transmissions is generally defined by a lower cutoff frequency of approximately 30 kilohertz, and a higher cutoff frequency which varies depending upon the particular technology. Since the POTS and xDSL signals are defined by isolated frequency bands, both signals may be transmitted over the same two-wire loop.
Indeed, twisted pair public telephone lines are increasingly being used to carry relatively high-speed signals instead of, or in addition to, telephone signals. Examples of such signals are ADSL (asymmetric digital subscriber line), HDSL (High Density Subscriber Line, T1(1.544 Mb/s), and ISDN signals. There is a growing demand for increasing use of telephone lines for high speed remote access to computer networks, and there have been various proposals to address this demand, including using voice over data systems to communicate signals via telephone lines at frequencies above the voice-band. The provision in the public telephone network of varied services using such diverse communications systems imposes a requirement that different and similar systems not interfere with one another.
In the telecommunication art, the term “crosstalk” refers to interference that enters a message channel from one or more other channels through a path coupling the message channel with the interfering channels. Crosstalk can create annoyance in a voice system or errors in a data system. Crosstalk's impact depends upon such factors as the listeners hearing acuity, the extraneous noise, the frequency response characteristic of the coupling path, and the level of the disturbing signal.
There are generally two types of crosstalk mechanisms that are characterized, one being NEXT (near-end crosstalk) and the other being FEXT (far-end crosstalk). NEXT results from a disturbing source connected at one end of the wire pair causing interference in the message channel at the same end as the disturbing source. FEXT is that portion of the disturbing signal propagated to far end of the message channel, that is, the end opposite to the end to which the disturbing source is connected.
Allocations of wire pairs within telephone cables in accordance with service requests have typically resulted in a random distribution of pair utilization with few precise records of actual configurations. In addition, due to the nature of pair twisting in cables, and where cable branching and splicing occurs, a wire pair can be in close proximity to different other pairs over different parts of its length. At a telephone CO (central office), pairs in close proximity may be carrying diverse types of service using various modulation schemes, with considerable differences in signal levels (and receiver sensitivities) especially for pairs of considerably different lengths.
Statistical data has been developed that can be used to estimate crosstalk between services using different pairs of multi-pair telephone cables, for example in terms of BER (bit error rate) based on power spectral density (PSD, for example measured in milliwatts per Hertz expressed in decibels, or dBm/Hz) overlap between the services. However, this statistical data is of limited use in practice in the provision of a new service using equipment connected to a specific wire pair, in view of factors such as those discussed above.
A common feature of the above-mentioned transmission systems is that twisted-pair phone lines are used as at least a part of the transmission medium that connects a central office (e.g., telephone company) to users (e.g., residence or business). It is difficult to avoid twisted-pair wiring from all parts of the interconnecting transmission medium. Even though fiber optics may be available from a central office to the curb near a user's residence, twisted-pair phone lines are used to bring in the signals from the curb into the user's home or business.
The twisted-pair phone lines are grouped in a binder. While the twisted-pair phone lines are within the binder, the binder provides reasonably good protection against external electromagnetic interference. However, within the binder, the twisted-pair phone lines induce electromagnetic interference on each other. This type of electromagnetic interference is generally known as crosstalk interference, which includes NEXT interference and FEXT interference. As the frequency of transmission increases, the crosstalk interference becomes substantial. As a result, the data signals being transmitted over the twisted-pair phone lines at high speeds can be significantly degraded by the crosstalk interference caused by other twisted-pair phone lines in the binder. The problem worsens as the speed of the data transmission increases or as the loop length increases.
As is known, NEXT interference may result from several sources. One such source may be referred to upstream “leak” levels into a DSL downstream service at the customer premise (CP) side. As is also known, there are generally two techniques or approaches to reduce NEXT upstream leak: blind and supervised. Blind source separation is the separation of a composite signal into its constituent component signals, without a priori knowledge of those signals. A growing number of researchers have published articles discussing techniques that perform blind source separation (BSS). For example, “Equivariant Adaptive Source Separation”, JF CARDOSO & B. LAHELD IEEE Trans. on Signal Proc., VOL. 44, No. 12, December 1996, and “Adaptive Blind Source Separation for virtually any Probability Density Function”, V. ZARZOSO, A. NANDI. IEEE Trans. on Signal processing, Vol. 48, No. 2 Feb. 2000, discuss blind source separation. Both of these articles are hereby incorporated by reference in their entireties. These techniques find use in various applications, including crosstalk removal in multichannel communications, multipath channel identification, and equalization. Many of the BSS techniques require (or assume) a statistical independence between the component signals to accurately separate the signals. Additional theoretical progress in signal modeling has generated new techniques that address the problem of identifying statistically independent signals, which is a recognized problem that lies at the heart of source separation.
In contrast, “supervised” techniques rely on other information sources, collected from physical sensors. For example, “Multichannel Signal Processing for Data Communications in the Presence of Crosstalk”, IEEE trans. on Communications, VOL. 38, No. 4, April 1990, and “Frequency selective NEXT Cancellation”, J. CIOFFI, ICASSP 2000, VOL. 5, pp. 2841–2844, discuss supervised techniques. Both of these articles are hereby incorporated by reference in their entireties.
Accordingly, there is a need to provide an improved system and method for canceling crosstalk interference.